U.S. patent number 10,625,000 [Application Number 15/390,056] was granted by the patent office on 2020-04-21 for methods and devices for controlling negative pressure at a wound site.
This patent grant is currently assigned to Paul Hartmann AG. The grantee listed for this patent is Paul Hartmann AG. Invention is credited to Pierre Croizat, Chris Dawber, Jurgen Hofstetter, Mark Hsieh, James Stein.
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United States Patent |
10,625,000 |
Croizat , et al. |
April 21, 2020 |
Methods and devices for controlling negative pressure at a wound
site
Abstract
The invention relates to a method for estimating a negative
pressure at a wound site during a negative pressure wound therapy.
The method comprises the steps of determining a negative pressure
value by means of a pressure sensor, determining a pump speed
associated with the electrical pump, multiplying the pump speed by
a constant to obtain a modification value, and combining said
modification value with the negative pressure value determined by
means of the pressure sensor to obtain a modified negative pressure
value. Said modified negative pressure value corresponds to the
estimated negative pressure present at the wound site. The
invention further relates to a negative pressure wound therapy
system adapted to execute said method of estimating a negative
pressure at a wound site.
Inventors: |
Croizat; Pierre
(Herbrechtingen, DE), Hofstetter; Jurgen (Heldenheim,
DE), Stein; James (Cambridge, GB), Dawber;
Chris (Cambridge, GB), Hsieh; Mark (Cambridge,
GB) |
Applicant: |
Name |
City |
State |
Country |
Type |
Paul Hartmann AG |
Heidenheim |
N/A |
DE |
|
|
Assignee: |
Paul Hartmann AG (Heidenheim,
DE)
|
Family
ID: |
55027582 |
Appl.
No.: |
15/390,056 |
Filed: |
December 23, 2016 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20170189588 A1 |
Jul 6, 2017 |
|
Foreign Application Priority Data
|
|
|
|
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Dec 30, 2015 [EP] |
|
|
15203110 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61M
1/0088 (20130101); A61M 1/0031 (20130101); A61M
1/0025 (20140204); A61M 2205/3365 (20130101); A61M
2205/3344 (20130101); A61M 2205/583 (20130101); A61M
2205/52 (20130101); A61M 2205/3382 (20130101); A61M
2205/505 (20130101); A61M 2205/15 (20130101); A61M
2205/50 (20130101); A61M 2205/18 (20130101); A61M
2205/3331 (20130101); A61M 2205/3337 (20130101) |
Current International
Class: |
A61M
1/00 (20060101); A61M 27/00 (20060101); A61F
13/00 (20060101); A61F 13/02 (20060101); A61K
9/22 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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102009038130 |
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Feb 2011 |
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DE |
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102009038131 |
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Feb 2011 |
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DE |
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102011075844 |
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Nov 2012 |
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0777504 |
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Oct 1998 |
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EP |
|
1095465 |
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Apr 2008 |
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EP |
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1863549 |
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Feb 2012 |
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EP |
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2464393 |
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Sep 2015 |
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EP |
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2464394 |
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Sep 2015 |
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EP |
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2008039314 |
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Apr 2008 |
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WO |
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2009047524 |
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Apr 2009 |
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WO |
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2010072349 |
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Jul 2010 |
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WO |
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2012156174 |
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Nov 2012 |
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WO |
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2014177544 |
|
Nov 2014 |
|
WO |
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2014177545 |
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Nov 2014 |
|
WO |
|
Primary Examiner: Zalukaeva; Tatyana
Assistant Examiner: Treyger; Ilya Y
Attorney, Agent or Firm: Dilworth & Barrese, LLP
Claims
The invention claimed is:
1. A negative pressure wound therapy system, comprising: an
electrical pump to generate negative pressure; a tachometer
configured to determine a pump speed associated with the electrical
pump; a first fluid path fluidly connectable to a wound site and to
the electrical pump such that the wound site is subjected to a
negative pressure via the electrical pump; a pressure sensor
configured to determine a negative pressure value, wherein the
pressure sensor is located in the first fluid path between the
wound site and the electrical pump; an input device to adjust
settings on the negative pressure wound therapy system; a
controller configured to execute the following steps: (i) multiply
the determined pump speed by a constant to obtain a modification
value; and (ii) combine the modification value with the determined
negative pressure value to obtain a modified negative pressure
value, wherein the modified negative pressure value corresponds to
an estimated negative pressure present at the wound site.
2. A negative pressure wound therapy system according to claim 1,
wherein when the negative pressure wound therapy system is in an
active state, the controller is further adapted to continuously or
intermittently execute the steps (i) and (ii).
3. The negative pressure wound therapy system according to claim 1,
wherein the constant is a value selected from the range of 0.0025
mmHg/RPM to 0.0225 mmHg/RPM.
4. The negative pressure wound therapy system according to claim 3,
wherein the range is between of 0.00375 mmHg/RPM to 0.015
mmHg/RPM.
5. The negative pressure wound therapy system according to claim 4,
wherein the value of the constant is 0.0075 mmHg/RPM.
6. The negative pressure wound therapy system according to claim 1,
wherein the electrical pump is a membrane pump.
7. The negative pressure wound therapy system according to claim 1,
wherein the input device is a touch screen.
8. The negative pressure wound therapy system according to claim 1,
which further comprises a canister for collecting fluid from the
wound site.
9. The negative pressure wound therapy system according to claim 1,
which further comprises a relief valve for venting the negative
pressure wound therapy system.
10. The negative pressure wound therapy system according to claim
9, which further comprises a second fluid path fluidly connectable
to the wound site and the relief valve, wherein the first fluid
path and the second fluid path are in fluid communication at the
wound site.
Description
FIELD OF THE INVENTION
The invention relates to control methods for a negative pressure
wound therapy system. In particular, the invention relates to a
method of estimating a negative pressure at a wound site during a
negative pressure wound therapy. Moreover, the invention relates to
a negative pressure wound therapy system adapted to execute the
wound pressure estimation method according to the invention.
BACKGROUND
Negative pressure wound treatment devices (vacuum wound treatment
devices) have been described many times, in particular, in US
2004/0073151 A1, WO 2009/047524 A2, EP 1 905 465 A1, WO 2008/039314
A2 or EP 777 504 B1 as well as in EP 1 863 549 B1, EP 2 464 394 A1,
WO 2012/156174 A1 or EP 2 464 393 A1 of the assignee.
In devices of this type for negative pressure wound therapy (npwt),
a suction pump (sometimes incorrectly called "vacuum pump")
communicates with the wound or the wound area via a suction line,
wherein a wound dressing and an air-tight cover material is
provided for air-tight sealing of the wound and the wound area,
such that a negative pressure can be generated in the wound region
and fluids can be extracted by suction from the wound region.
The term negative pressure in connection with the present invention
defines an air pressure that is lower than the ambient air pressure
(atmospheric air pressure). The cover material of a wound dressing
for air-tight sealing of a wound region must therefore be designed
in such a fashion that it withstands the pressure difference that
is established such that a negative pressure can actually be
applied to and maintained in the wound region. The wound dressing
and the cover material are, however, typically flexible to a
certain degree. In the field of negative pressure therapy for the
treatment of wounds, the negative pressure is quantitatively
defined as the pressure difference between ambient air pressure and
the air pressure applied below the cover material. In the field of
negative pressure therapy, this pressure difference is typically at
most 250 mmHg (mm mercury column) (1 mm Hg=1 Torr=133.322 Pa). This
negative pressure range of up to maximally 250 mmHg has turned out
to be suitable for wound healing. A preferred negative pressure
range is between 10 and 150 mmHg.
For typical negative pressure treatment, the negative pressure that
is applied to the wound using the device can either be kept
substantially constant with time or can be varied with time, in
particular in cycles which can be realized by a correspondingly
designed and programmed control device for the negative
pressure-generating device, in particular in dependence on further
parameters.
An advantageous flexible suction line, e.g. in the form of a
drainage hose, is provided for applying a negative pressure and
advantageously also for extracting body fluids, the drainage hose
communicating at one end with the wound area or the wound region
via a so-called port in the area of the wound cover material, and
at the other end communicating with a container for receiving the
sucked body fluids or with the negative pressure generating
device.
In addition to negative pressure wound treatment, the present
invention may also be used for other applications for providing a
negative pressure for medical applications, in particular,
extraction of any body fluids by suction, in the field of medical
incontinence management, in the field of care of stoma patients or
in the field of extraction of wound exudates, if necessary, thereby
using rinsing liquids and also without application of a negative
pressure over considerable time periods.
In general, negative pressure wound therapy apparatuses are
available as stationary or as portable devices. The
before-mentioned definition "portable device" means that the
patient can carry the device along so that he/she is mobile and
his/her wound can nevertheless be permanently treated, i.e. without
interruption. The portable device may thereby be held on the body
of the patient and be carried along by means of any fastening
means, for example in the form of a flexible belt or a shoulder
strap. A portable device of the above-mentioned type naturally may
also be used for stationary operation, i.e. detached from the body
of the patient. In this case, it may e.g. be mounted to a hospital
bed or be deposited next to the hospital bed.
SUMMARY OF THE INVENTION
Up-to-date negative pressure wound therapy devices are usually
capable of managing different therapy situations. The devices can
accommodate complex treatment procedures set by the user. This is
achieved by a microprocessor based control system, which integrates
inputs, such as the user settings or sensor signals, and converts
it into outputs, such as suction pump control signals, vent control
signals, alarm signals or display messages. The user of the device
predetermines a target pressure to be applied at the wound by
entering the treatment parameters into the user interface of the
apparatus. The control system is programmed to generate and
maintain the target pressure in its internal fluid system which is
in fluid communication with the wound. The target pressure may be a
constant negative pressure or a varying negative pressure. In order
to avoid any discrepancies between the negative pressure
predetermined by the therapy scheme and the pressure actually
applied to the wound tissue, it is necessary to monitor the
pressure permanently. The pressure measurements serve as an input
for the control system of the device to adjust pump activity
accordingly. Typically, a negative pressure device has a pressure
sensor, which is present inside of the housing of the apparatus. In
this case, the pressure sensor is very close to the negative
pressure source (pump) and remote from the wound space. However, a
pressure sensor located near the pressure source will signal a
higher negative pressure (i.e. the difference between the measured
pressure and the surrounding atmosphere is higher) than the
negative pressure that is actually present at the wound, because
there is a pressure gradient through the fluid system. In
particular, a drop between the pressure source and the wound space
appears.
Measuring the accurate pressure at the wound tissue can only be
accomplished by a pressure sensor at or near the wound space. It is
also known from the prior art to connect a pressure sensor present
inside of the negative pressure unit (i.e. near the pressure
source) with the wound space by a separate "pressure monitoring
tube". However, locating an air pressure sensor at the wound site
as well as providing an additional pressure monitoring tube
increases complexity and costs of the negative pressure therapy
system.
Based on a device for providing a negative pressure for medical
applications, whereby the device has a pressure sensor located near
the pressure source, is the underlying purpose of the present
invention to further improve therapy. In particular, it is desired
to optimize accuracy and operational safety of the device. The
therapy device should be able to implement a predetermined therapy
scheme exactly and in a reproducible manner. Any discrepancies
between the predetermined pressure and the pressure actually
applied to the wound should be minimized.
A solution for the aforementioned problems is provided by the
present invention. The inventors of the present invention found a
novel wound pressure estimation method, which provides a suitable
alternative to directly measuring the pressure at the wound
site.
According to a first aspect the invention, a method of estimating a
negative pressure at a wound site during a negative pressure wound
therapy is proposed. Said method, which is designated in the
present specification as the "wound pressure estimation method",
comprises the following steps: i. determining a negative pressure
value by means of a pressure sensor, wherein the pressure sensor is
located in a fluid path between the wound site and an electrical
pump, said electrical pump being used for generating a negative
pressure, ii. determining a pump speed associated with the
electrical pump, iii. multiplying the pump speed by a constant to
obtain a modification value, iv. combining said modification value
with the negative pressure value determined by means of the
pressure sensor to obtain a modified negative pressure value
corresponding to the estimated negative pressure present at the
wound site.
Usually, the negative pressure value (step i.) and the pump speed
(step ii.) are determined at the same time by means of, for
example, the pressure sensor and the tachometer of the negative
pressure wound therapy system carrying out the wound pressure
estimation method.
The wound pressure estimation method works sufficiently well for
practical purposes. When comparing the pressure values produced by
the novel estimation method with the actual pressure at the wound
site (determined under experimental conditions and by using an
additional pressure sensor at the wound site) only minor
discrepancies were detected.
The second aspect of the invention pertains to a negative pressure
wound therapy system. The negative pressure wound therapy system
according to the second aspect of the invention comprises an
electrical pump for generating negative pressure, optionally a
tachometer for determining a pump speed associated with the
electrical pump, a pressure sensor for determining negative
pressure values, a controller for controlling activity of the
electrical pump, input means for adjusting settings on the negative
pressure wound therapy system, said input means being operable by
the user of the negative pressure wound therapy system, and a first
fluid path fluidly connectable to a wound site and to the
electrical pump such that the wound site can be subjected to a
negative pressure. The pressure sensor is located in the first
fluid path between the wound site and the electrical pump. The
negative pressure wound therapy system according to the second
aspect of the invention is characterized in that the controller of
the negative pressure wound therapy system is adapted to execute a
method according to the first aspect of the invention.
A negative pressure wound therapy system having a pressure
controller which is adapted to execute the wound pressure
estimation method works reliably and accurately. The system
implements any predetermined negative pressure wound treatment
schemes in a reproducible manner and under all typical medical
treatment situations. Pressure estimation is not impaired by wound
size or by extensive amounts of wound exudate. A negative pressure
wound therapy system according to the second aspect of the
invention can be designed robustly and simply, because additional
components (such as an additional pressure sensor or an additional
fluid system for pressure monitoring) are not required.
DEFINITIONS
As explained previously, the term "negative pressure" as used in
connection with the present invention, defines an air pressure that
is lower than the ambient air pressure (atmospheric air pressure).
In the field of negative pressure therapy of wounds, the negative
pressure is quantitatively defined as the pressure difference
between ambient air pressure and the air pressure within the fluid
path of the npwt system, in particular the air pressure applied
below the cover material of the npwt dressing. For example, a
negative pressure of 125 mmHg determined by means of a pressure
sensor located in the fluid path of an npwt system indicates that
the pressure at the pressure sensor location has been reduced by
125 mmHg compared to the ambient air pressure surrounding the npwt
system. In general, negative pressure values are provided with a
positive algebraic sign in this specification.
In general, a pressure gradient (.DELTA.P/.DELTA.t or dP/dt)
indicates a change in pressure which occurs in a certain period of
time. For example, a negative pressure gradient of 2 mmHg/s may
indicate an increase of negative pressure with a rate of 2 mmHg per
second.
A target negative pressure is a negative pressure value selected by
the user of the npwt system. Accordingly, the target negative
pressure indicates the negative pressure value, which should be
established during wound treatment. Preferably, target negative
pressure values between 10 mmHg and 150 mmHg are used for negative
pressure wound therapy.
Similarly, a target negative pressure gradient indicates a negative
pressure gradient which should be established during wound
treatment.
A negative pressure error is a pressure difference between two
pressure values, for example between a measured negative pressure
value and a target negative pressure value. In general, a pressure
difference is calculated by performing a subtraction of the
corresponding pressure values.
Similarly, a negative pressure gradient error is a difference
between two pressure gradient values, for example between a
measured negative pressure gradient and a target negative pressure
gradient. Again, the difference is generally calculated by
performing a subtraction of the corresponding pressure gradient
values.
The controller of the negative pressure wound therapy system
according to the second aspect of the invention is adapted to
execute a method according to the first aspect of the invention.
This means that the controller is not only capable of executing the
method (e.g. by having the required processing power and memory),
but also actually applies the method when the negative pressure
wound therapy system is used for wound treatment. This requires
that the controller is programmed to perform the algorithm of the
method according to the first aspect of the invention.
PREFERRED EMBODIMENTS OF THE INVENTION
The value of the constant may vary between different negative
pressure wound therapy systems. In particular, the performance and
the size of the pump, the length of the suction tube, the diameter
of the suction tube and (to a minor extent) also the material of
the suction tube may influence the value of the constant. However,
in a preferred embodiment of the wound pressure estimation method,
the constant is a value selected from the range of 0.0025 mmHg/RPM
to 0.0225 mmHg/RPM. In an even more preferred embodiment of the
wound pressure estimation method, the constant is a value selected
from the range of 0.00375 mmHg/RPM to 0.015 mmHg/RPM. In
particular, the constant of the wound pressure estimation method
may have a value of approximately 0.0075 mmHg/RPM.
According to a preferred embodiment of the wound pressure
estimation method, the step of combining the modification value
with the negative pressure value determined by means of the
pressure sensor comprises or consists of subtracting the
modification value from the negative pressure value determined by
means of the pressure sensor. In this embodiment, the algebraic
sign of the constant is usually positive. Otherwise, instead of a
subtraction an addition would be performed and the negative
pressure at the wound site would be estimated incorrectly.
The modified negative pressure value may be used by the controller
of the negative pressure wound therapy system to establish the
desired target negative pressure at the wound site. Therefore, the
wound pressure estimation method may comprise a further step or may
be combined with a further step in which the modified negative
pressure value is used by a controller of the negative pressure
wound therapy system to establish a target negative pressure at the
wound site. In particular, the wound pressure estimation method may
comprise a further step or may be combined with a further step in
which the modified negative pressure value is used as an input
variable for a pressure control method (e.g. the first or the
second pressure control method disclosed herein). The pressure
control method is executed by a controller of the negative pressure
wound therapy system in order to establish a target negative
pressure at the wound site.
According to a preferred embodiment of the second aspect of the
invention, the controller of the negative pressure wound therapy
system is adapted to execute the method according to the first
aspect of the invention such that the negative pressure wound
therapy system in its active state continuously or intermittently
executes the method according to the first aspect of the invention.
The negative pressure wound therapy system is in an active state
when it is switched on, in particular when the negative pressure
wound therapy system applies negative pressure to a wound site or
maintains negative pressure at a wound site. Normally, the active
state ends if the negative pressure wound therapy system is
switched off. The active state may also end if an alarm situation
such as a blockage condition, a canister full condition or a
leakage condition occurs.
First Pressure Control Method
According to a preferred embodiment of the invention, the method
according to the first aspect of the invention is used in
combination with a first method for generating a negative pressure
at a wound site during a negative pressure wound therapy. This
first method is designated in the present specification as the
"first pressure control method" and comprises the following steps:
i. setting a target negative pressure on a negative pressure wound
therapy system, said negative pressure wound therapy system being
used for the negative pressure wound therapy, ii. determining a
negative pressure value by means of a pressure sensor, wherein the
pressure sensor is located in a fluid path between the wound site
and an electrical pump, said electrical pump being used for
generating a negative pressure, iii. calculating a difference
between the determined negative pressure value and the target
negative pressure setting to obtain a negative pressure error, iv.
determining a target negative pressure gradient by means of a first
function, wherein the first function maps the negative pressure
error to the target negative pressure gradient, v. adjusting a
control signal for the electrical pump in response to the value of
the target negative pressure gradient, said control signal controls
the speed of the electrical pump.
The first pressure control method is typically executed in a
control loop.
According to a particularly preferred version of the first pressure
control method, the first function essentially exhibits a curve
progression as shown in FIG. 4.
According to a further preferred version of the first pressure
control method, the control signal for the electrical pump is a
signaling voltage or a pulse-width modulation signal.
Second Pressure Control Method
According to an even more preferred embodiment of the invention,
the method according to the first aspect of the invention is used
in combination with a second method for generating a negative
pressure at a wound site during a negative pressure wound therapy.
This second method is designated in the present specification as
the "second pressure control method" and comprises the following
steps: i. setting a target negative pressure on a negative pressure
wound therapy system, said negative pressure wound therapy system
being used for the negative pressure wound therapy, ii. determining
a negative pressure value by means of a pressure sensor, wherein
the pressure sensor is located in a fluid path between the wound
site and an electrical pump, said electrical pump being used for
generating a negative pressure, iii. calculating a difference
between the determined negative pressure value and the target
negative pressure setting to obtain a negative pressure error, iv.
determining a target negative pressure gradient by means of a first
function, wherein the first function maps the negative pressure
error to the target negative pressure gradient, v. determining an
actual negative pressure gradient, vi. calculating a difference
between the actual negative pressure gradient and the target
negative pressure gradient to obtain a negative pressure gradient
error, vii. determining an integrator input by means of a second
function, wherein the second function maps the negative pressure
gradient error to the integrator input, viii. determining a control
signal for the electrical pump comprising the use of an integrator,
said integrator processes the integrator input and said control
signal controls the speed of the electrical pump.
The second pressure control method is typically executed in a
control loop.
According to preferred version of the second pressure control
method, the actual negative pressure gradient is determined based
on a first and on a second negative pressure value (resulting from
a negative pressure measurement). The first negative pressure value
is determined by means of the pressure sensor prior to step ii. (of
the second pressure control method). The second negative pressure
value is the negative pressure value of step ii. (of the second
pressure control method). The second pressure control method is
typically executed in a control loop. The aforementioned first
negative pressure value may originate from a first cycle
(determined in step ii. of this first cycle), wherein the
aforementioned second negative pressure value may originate form a
subsequent, second cycle (determined in step ii. of this second
cycle). Generally, the aforementioned first negative pressure value
may originate from an earlier cycle as the second negative pressure
value.
According to a particularly preferred version of the second
pressure control method, the first function essentially exhibits a
curve progression as shown in FIG. 4 and/or the second function
essentially exhibits a curve progression as one of the functions
shown in FIG. 5 b.
According to another preferred version of the second pressure
control method, the control signal for the electrical pump is a
signaling voltage or a pulse-width modulation signal.
Typically, the integrator used for the second pressure control
method processes the integrator input to an integrator output. The
integrator output may be used as the control signal for the
electrical pump. Alternatively, the control signal for the
electrical pump may be derived from the integrator output by
applying further processing steps. The integrator per se as well as
its mode of action is known in the prior art, for example as a part
of common PID controllers. In principle, the integrator
continuously calculates the sum of the integrator input values of
consecutive cycles of the second pressure control method. For
example, when performing the second pressure control method over
four consecutive cycles (the second pressure control method is
typically executed by the npwt controller in a loop) integrator
input values of 0.05, 0.1, 0.15 and -0.05 may result. Consequently,
the integrator output in this example would be calculated as
follows: Integrator output=0.05+0.1+0.15+(-0.05)=0.25
First Blockage Detection Method
According to another advantageous embodiment of the invention, the
method according to the first aspect of the invention is used in
combination with a first method for detection of blockages
appearing in the fluid system. Said first method, which is
designated in the present specification as the "first blockage
detection method", comprises the following steps: i. generating a
negative pressure at a wound site by means of an electrical pump,
ii. recording the negative pressure, iii. venting the fluid path of
the negative pressure wound therapy system by opening a relief
valve, wherein during the ventilation the electrical pump is
stopped, iv. determining and recording a negative pressure drop
during the ventilation step, wherein the negative pressure drop is
determined for a predetermined period of time, v. optionally
closing the relief valve when the predetermined period of time has
elapsed, wherein closing the relief valve finalizes the ventilation
step, vi. generating a blockage signal in a controller of the
negative pressure wound therapy system if the negative pressure
drop observed during the ventilation step (over the predetermined
period of time) is less than a predetermined negative pressure
drop.
Preferably, the negative pressure wound therapy system executes the
first blockage detection method every 1 to 10 minutes, in
particular every 5 minutes, during the negative pressure wound
therapy. The blockage signal may be immediately communicated to the
user of the npwt system, for example by means of an acoustic and/or
visual alarm. Negative pressure wound therapy systems usually
comprise a speaker and a display which may be used to generate such
alarms. Instead of communicating the blockage signal immediately to
the user, it might be advantageous to repeat the first blockage
detection method (e.g. after 1 to 10 minutes, in particular after 2
minutes). Only if the repetition confirms the blocked condition,
the alarm is generated.
Preferably, the negative pressure drop is determined by determining
a difference between the negative pressure of step ii. of the first
blockage detection method and the negative pressure present in the
negative pressure wound therapy system when the predetermined
period of time has elapsed. Preferably, the calculated difference
is related to the negative pressure of step ii. of the first
blockage detection method to obtain a percentage negative pressure
drop. For example, a percentage negative pressure drop of 10% is
obtained, if the negative pressure of step ii. is 100 mmHg and the
negative pressure at the end of the predetermined period of time is
90 mmHg. The corresponding formula for this example can be
summarized as follows: Percentage negative pressure drop=((100
mmHg-90 mmHg)/100 mmHg).times.100=10%
The general formula is: negative pressure drop [in %]=((negative
pressure of step ii.)-(negative pressure at the end of the
predetermined period))/negative pressure of step ii.).times.100
It is particularly preferred that the predetermined period of time
is a value selected from the range of 20 seconds to 120 seconds. It
is even more preferred that the predetermined period of time is a
value selected from the range of 30 seconds to 60 seconds. It was
found that a predetermined period of approximately 45 seconds is
most preferred.
Preferably, the predetermined negative pressure drop is a relative
(percentaged) value. Therefore, in a preferred version of the first
blockage detection method, the blockage signal is generated in the
controller if the negative pressure drop observed during the
ventilation step is less than the predetermined negative pressure
drop having a value selected from the range of 10% to 30% compared
to the negative pressure of step ii. (of the first blockage
detection method). In particular, the predetermined negative
pressure drop may have a value of approximately 20% compared to the
negative pressure of step ii. (of the first blockage detection
method).
The first blockage detection method preferably further comprises
eliminating the blocked condition after a blockage signal has been
generated by the controller. The blocked condition is usually
eliminated by the user of the negative pressure wound therapy
system, i.e. for example a patient or a caregiver. To eliminate the
blocked condition, the user has to, for example, replace the
clogged suction conduit which causes the blocked condition.
Second Blockage Detection Method
According to another even more advantageous embodiment of the
invention, the method according to the first aspect of the
invention is used in combination with a second method for detection
of blockages appearing in the fluid system. Said second method,
which is designated in the present specification as the "second
blockage detection method", comprises the following steps: i.
generating a negative pressure at a wound site by means of an
electrical pump, ii. recording the negative pressure, iii. venting
the fluid path of the negative pressure wound therapy system by
opening a relief valve, wherein during the ventilation the
electrical pump is stopped, iv. closing the relief valve when the
negative pressure has dropped by a predetermined value, wherein
closing the relief valve finalizes the ventilation step, v.
determining and recording a negative pressure gradient occurring
during the entire ventilation step (average negative pressure
gradient during the ventilation step), vi. reactivating the
electrical pump to reestablish the negative pressure of step ii.,
vii. determining and recording a number of pump turns, which are
required to reestablish the negative pressure of step ii., viii.
generating a first or a second blockage detection data set, said
first or said second blockage detection data set comprising the
recorded negative pressure of step ii., the recorded negative
pressure gradient occurring during the entire ventilation step and
the recorded number of pump turns, which were required to
reestablish the negative pressure of step ii., ix. executing a
classification algorithm which allows to discriminate a first
blockage detection data set, said first blockage detection data set
being correlated to an unblocked condition of the negative pressure
wound therapy system, from a second blockage detection data set,
said second blockage detection data set being correlated to a
blocked condition of the negative pressure wound therapy
system.
The number of pump turns in step vii. may be derived from pump
speed measurements.
If the recorded negative pressure of step ii., the recorded
negative pressure gradient and the recorded number of pump turns
are directly used for the classification algorithm, the step of
generating a first or a second blockage detection data set only
consists of a compilation of these variables to form a single
(first or second) data set, said (first or second) data set being
used for the classification algorithm. In this case, the step of
generating the first or the second blockage detection data set does
not necessarily have to include any further activity of the npwt
system (i.e. the controller) since the values of the three
aforementioned variables have already been recorded by the
system.
Preferably, the recorded negative pressure of step ii., the
recorded negative pressure gradient and/or the recorded number of
pump turns may be mathematically processed as will be explained
more in detail below. According to these preferred embodiments, the
step of generating the blockage detection data set may include
further mathematical operations. Each first or second blockage
detection data set which has been generated by using the recorded
negative pressure of step ii., the recorded negative pressure
gradient and the recorded number of pump turns, is a first or a
second blockage detection data set according to the present
invention (irrespective whether these variables are further
mathematically processed and/or combined with each other or
not).
It is recommended to express the predetermined negative pressure
value as a relative (percentaged) value. According to a preferred
version of the second blockage detection method, the relieve valve
closes when the negative pressure has dropped by the predetermined
value selected from the range of 10% to 30% compared to the
negative pressure of step ii. of the second blockage detection
method. In particular, the relieve valve closes when the negative
pressure has dropped by the predetermined value of approximately
20% compared to the negative pressure of step ii. of the second
blockage detection method. In order to close the valve immediately
after the predetermined pressure drop has occurred (by for example
20%), the negative pressure wound therapy system monitors the
negative pressure by permanently performing pressure measurements.
These pressure measurements may also be used to determine a
pressure gradient. The following example refers to the pressure
gradient of step v.
Determining the negative pressure gradient may include comparing a
first pressure measurement at the start of the ventilation step
(typically the recorded negative pressure of step ii.) and a second
pressure measurement at the end of the ventilation step. For
example, the first pressure measurement may determine a negative
pressure value of 100 mmHg and the second pressure measurement may
determine a negative pressure value of 80 mmHg, wherein the
measurements have been determined in a time interval of 10 seconds.
The negative pressure gradient in this example then amounts to -2
mmHg/s. The negative algebraic sign of the negative pressure
gradient can be used to indicate that the gradient is related to a
negative pressure drop. The corresponding formula for this example
can be formulated as follows: Negative pressure gradient: (80
mmHg-100 mmHg)/10 seconds=-2 mmHg/s
According to an even more advanced version of the second blockage
detection method the system monitors whether the negative pressure
drop is actually accomplished within a predetermined period of
time. This predetermined period of time in the second blockage
detection method may have a value selected from the range of 20
seconds to 120 seconds. Preferably, the range for the predetermined
period of time in the second blockage detection method is 30
seconds to 60 seconds. In particular, the predetermined period of
time in the second blockage detection method is approximately 45
seconds. If the negative pressure drop is not accomplished within
the predetermined period of time, this observation alone may
already be sufficient to determine a blockage condition. A blockage
signal may then immediately be generated in the controller of the
negative pressure wound therapy system (as suggested in the first
blockage detection method).
According to a preferred version of the second blockage detection
method, the first or the second blockage detection data set
comprises a variable x.sub.B, which corresponds to (or is derived
from) the recorded negative pressure of step ii. of the second
blockage detection method, a variable y.sub.B, which corresponds to
(or is derived from) the recorded negative pressure gradient
occurring during the entire ventilation step and a variable
z.sub.B, which corresponds to (or is derived from) the recorded
number of pump turns that were required to reestablish the negative
pressure of step ii. of the second blockage detection method.
According to an even more preferred version of the second blockage
detection method, the variable y.sub.B is derived from the recorded
negative pressure gradient by relating the recorded negative
pressure gradient to a negative pressure value obtained by
calculating (0.5.times.(P.sub.S+P.sub.D)). P.sub.S is or
corresponds to the negative pressure of step ii. of the second
blockage detection method. P.sub.D is or corresponds to the
negative pressure at the end of the ventilation step. The formula
to calculate the variable y.sub.B can be described as follows:
y.sub.B=(negative pressure
gradient)/(0.5.times.(P.sub.S+P.sub.D))
If, for example, the negative pressure gradient is at -2 mmHg/s,
P.sub.S is at 100 mmHg and P.sub.D is at 80 mmHg, y.sub.B according
to this preferred embodiment amounts to -1/45 s.sup.-1. y.sub.B=(-2
mmHg/s)/(0.5.times.(100 mmHg+80 mmHg))=(-1/45) s.sup.-1
The variable y.sub.B according to this preferred embodiment may be
regarded as a negative pressure gradient which is subjected to a
mathematical transformation to obtain a "relative negative pressure
gradient". Such a mathematical transformation may improve the use
of data sets by a support vector machine (svm) algorithm. It is,
for example, possible to generate a flat and uniform separation
plane by said support vector machine using the mathematically
transformed data. Instead, the separation plane would comprise a
curved conformation if the very same data set had been used without
further processing (i.e. without the mathematical transformation).
The support vector machine and the separation plane may be part of
the classification algorithm as mentioned below in connection with
a particularly preferred version of the second blockage detection
method.
To further improve application of the support vector machine, the
variable z.sub.B may also be subjected to a mathematical
transformation. Therefore, according to a particularly preferred
version of the second blockage detection method, the variable
z.sub.B is derived from the recorded number of pump turns by
relating the recorded number of pump turns to the negative pressure
drop during the ventilation step (or in other words by relating the
recorded number of pump turns to the amount of the negative
pressure increase to reestablish the negative pressure prior the
ventilation step). The corresponding formula can be summarized as
follows: z.sub.B=(number of pump turns)/(P.sub.S-P.sub.D)
Again, P.sub.S is or corresponds to the negative pressure of step
ii. of the second blockage detection method and P.sub.D is or
corresponds to the negative pressure at the end of the ventilation
step. If, for example, the number of pump turns amounts to 20,
P.sub.S amounts to 100 mmHg (which corresponds to the negative
pressure to which the system is initially regulated after the
ventilation step and up to which the number of pump turns is
recorded) and P.sub.D amounts to 80 mmHg, z.sub.B according to this
preferred embodiment possesses a value of 1 mmHg.sup.-1.
z.sub.B=20/(100 mmHg-80 mmHg)=1 mmHg.sup.-1
Therefore, the transformed variable z.sub.B in the aforementioned
example indicates that an average of 1 pump turn per mmHg of
negative pressure was required to reestablish the negative pressure
of step ii. of the second blockage detection method.
According to an advantageous version of the second blockage
detection method, the classification algorithm includes a support
vector machine to generate a hyperplane. In other words, the
classification algorithm preferably includes a hyperplane generated
by a support vector machine. The basic principle of a support
vector machine and a hyperplane is explained in more detail in the
part of the description relating to FIGS. 8 a to c.
In particular, the classification algorithm of the second blockage
detection method includes a support vector machine using a
three-dimensional space and a separation plane (hyperplane). This
separation plane may advantageously be a flat separation plane. In
the case of the three-dimensional support vector machine, the first
dimension of the three-dimensional space is preferably defined by
the recorded negative pressure of step ii. of the second blockage
detection method. The first dimension of the three-dimensional
space may also be a variable derived from the recorded negative
pressure of step ii. of the second blockage detection method. The
second dimension of the three-dimensional space is preferably
defined by the recorded negative pressure gradient during the
ventilation step. The second dimension of the three-dimensional
space may also be a variable derived from the recorded negative
pressure gradient during the ventilation step. Finally, the third
dimension of the three-dimensional space is preferably defined by
the recorded number of pump turns or a variable derived from the
recorded number of pump turns. Moreover, executing such a
classification algorithm preferably comprises evaluating the
blockage detection data set by determining whether the data point
in the three-dimensional space associated with the blockage
detection data set is on one or on the opposite side of the
separation plane. The separation plane separates data points
correlated to unblocked conditions from data points correlated to
blocked conditions of the negative pressure wound therapy
system.
According to another preferred version of the second blockage
detection method, a blockage signal in a controller of the negative
pressure wound therapy system is generated once the classification
algorithm detects a blocked condition. Preferably, the negative
pressure wound therapy system executes the second blockage
detection method every 1 to 10 minutes, in particular every 5
minutes, during the negative pressure wound therapy. As in the case
of the first blockage detection method, the blockage signal may be
immediately communicated to the user of the npwt system, for
example by means of an acoustic and/or visual alarm. Instead of
communicating the blockage signal immediately to the user, it might
be advantageous to repeat the second blockage detection method
(e.g. after 1 to 10 minutes, in particular after 2 minutes). The
alarm is generated only if the repetition confirms the blocked
condition. Typically, the negative pressure wound therapy system
according to the invention simultaneously executes the first and
the second blockage detection method. Depending on whether the
required pressure drop occurs in the pre-set time interval or not,
a blockage condition is detected by the system either by the first
or by the second blockage detection method.
The second blockage detection method preferably further comprises
eliminating the blocked condition after a blockage signal has been
generated by the controller. The blocked condition is usually
eliminated by the user of the negative pressure wound therapy
system, i.e. for example a patient or a caregiver. To eliminate the
blocked condition, the user possibly has to replace the clogged
suction conduit which causes the blocked condition.
Canister Full Detection Method
According to another very advantageous embodiment of the invention,
the method according to the first aspect of the invention is used
in combination with a method for detection of a canister full
condition. A canister full condition appears if the exudate
container of the negative pressure device is completely filled with
fluids sucked from the wound space such that its intake capacity is
exhausted. Said detection method is designated in the present
specification as the "canister full detection method" and comprises
the following steps, which are performed during a negative pressure
wound therapy: i. determining and recording a number of pump turns
associated with an electrical pump used for generating a negative
pressure in the negative pressure wound therapy system, wherein the
number of pump turns is determined (and recorded) for a
predetermined period of time, ii. determining and recording a
plurality of negative pressure values by means of a pressure
sensor, wherein the plurality of negative pressure values is
determined (and recorded) for the predetermined period of time,
iii. calculating and recording a negative pressure variation score
by means of the recorded negative pressure values of method step
ii., iv. generating a first or a second canister full detection
data set, said first or said second canister full detection data
set comprising the recorded number of pump turns and the recorded
negative pressure variation score, v. executing a classification
algorithm which allows to discriminate a first canister full
detection data set, said first canister full detection data set
being correlated to a canister not full condition of the negative
pressure wound therapy system, from a second canister full
detection data set, said second canister full detection data set
being correlated to canister full condition of the negative
pressure wound therapy system.
The number of pump turns in step i. may be derived from pump speed
measurements during the predetermined period of time. Normally, the
pump speed (step i.) and the negative pressure values (step ii.)
are determined for the same (predetermined) period of time, that is
the measurements for step i. and for step ii. are carried out
simultaneously. The plurality of negative pressure values will
usually comprise a high number of negative pressure values (for
example approximately 300 for 3 seconds) since electronic pressure
sensors usually work with a high pressure sampling rate and this
may improve the accuracy of the canister full detection method.
If the number of pump turns and the recorded negative pressure
variation score are directly used for the classification algorithm,
the step of generating a first or a second canister full detection
data set only consists of a compilation of these variables to form
a single (first or second) data set, said (first or second) data
set being used for the classification algorithm. In this case, the
step of generating the first or the second canister full detection
data set does not necessarily have to include any further activity
of the npwt system (i.e. the controller) since the values of the
two aforementioned variables have already been recorded by the
system.
It may however be advantageous to mathematically process the
recorded number of pump turns and/or the recorded negative pressure
variation score. Accordingly, the step of generating the first or
the second canister full detection data set may include further
mathematical operations. Each first or second canister full
detection data set, which has been generated as explained above, is
a canister full detection data set according to the present
invention (irrespective whether the variables pump turns and
pressure variation score are further mathematically processed
and/or combined with each other or not).
When using the novel canister full detection method disclosed
herein it has been found to be very advantageous when the
predetermined period of time is a value selected from the range of
1 second to 15 seconds. It is even more advantageous when the
predetermined period of time is a value selected from the range of
1 second to 6 seconds. In particular, the predetermined period of
time in the canister full detection method is approximately 3
seconds.
In general, the negative pressure variation score provides an
indication of the overall pressure change within the fluid-tight
sealed components of the npwt system during the predetermined
period of time. According to a preferred version of the canister
full detection method, the calculation of the negative pressure
variation score comprises the steps of i. calculating a plurality
of pressure differences by means of the negative pressure values
recorded during the predetermined period of time to obtain one or
more negative pressure increments and one or more negative pressure
decrements, wherein for calculating of each pressure difference
preferably two consecutively determined negative pressure values
are used, ii. calculating the sum of the negative pressure
increments to obtain a single value representing the degree of the
negative pressure increments, iii. calculating the sum of the
negative pressure decrements to obtain a single value representing
the degree of the negative pressure decrements, iv. calculating the
product of the single value representing the degree of the negative
pressure increments and of the single value representing the degree
of the negative pressure decrements, v. optionally extracting a
square root of the absolute value of the product calculated in
method step iv.
The following example illustrates the calculation of the negative
pressure variation score according to this preferred
embodiment:
Five negative pressure values are recorded during the predetermined
period of time, namely negative pressure value 1 (p.sub.1) having a
value of 120 mmHg, negative pressure value 2 (p.sub.2) having a
value of 110 mmHg, negative pressure value 3 (p.sub.3) having a
value of 115 mmHg, negative pressure value 4 (p.sub.4) having a
value of 125 mmHg, and negative pressure value 5 (p.sub.5) having a
value of 120 mmHg.
Four pressure differences can be calculated based on the five
negative pressure values, namely pressure difference 1 (pd.sub.1)
having a value of -10 mmHg (p.sub.2-p.sub.1), pressure difference 2
(pd.sub.2) having a value of 5 mmHg (p.sub.3-p.sub.2), pressure
difference 3 (pd.sub.3) having a value of 10 mmHg (p.sub.4-.sub.3),
and pressure difference 4 (pd.sub.4) having a value of -5 mmHg
(p.sub.5-p.sub.4).
As indicated by the algebraic signs, pd.sub.2 and pd.sub.3
represents negative pressure increments, wherein pd.sub.1 and
pd.sub.4 represents negative pressure decrements. Therefore, the
sum of the negative pressure increments (pd+) amounts to 15 mmHg
(pd.sub.2+pd.sub.3) and the sum of the negative pressure decrements
(pd-) amounts to -15 mmHg (pd.sub.1+pd.sub.4). The product (pdx) of
the sum of the negative pressure increments (pd+) and the sum of
the negative pressure decrements (pd-) amounts to -225 mmHg.sup.2
(pd+.times.pd-). Extracting the square root of the absolute value
of the product (pdx) yields the negative pressure variation score,
which in this example amounts to 15 mmHg ( {square root over
(|pdx|)}).
In a preferred version of the canister full detection method, the
first or the second canister full detection data set comprises a
variable x.sub.C. x.sub.C corresponds to (or is derived from) the
recorded number of pump turns. In addition, the first or the second
canister full detection data set according to this version
comprises another variable y.sub.C. y.sub.C corresponds to (or is
derived from) the recorded negative pressure variation score.
According to a particularly preferred version of the canister full
detection method, the classification algorithm includes a support
vector machine to generate a hyperplane. In other words, the
classification algorithm preferably includes a hyperplane generated
by a support vector machine.
In particular, the classification algorithm of the canister full
detection method includes a support vector machine using a
two-dimensional space and a separation line (hyperplane). This
separation line may advantageously be a linear separation line. In
the case of the two-dimensional support vector machine, the first
dimension of the two-dimensional space is preferably defined by the
recorded number of pump turns or a variable derived from the
recorded number of pump turns. The second dimension of the
two-dimensional space is preferably defined by the recorded
negative pressure variation score or a variable derived from the
recorded negative pressure variation score. Furthermore, executing
such a classification algorithm preferably comprises evaluating the
canister full detection data set by determining whether the data
point in the two-dimensional space associated with the canister
full detection data set is on one or on the opposite side of the
separation line. The separation line separates data points
correlated to canister not full conditions from data points
correlated to canister full conditions of the negative pressure
wound therapy system.
Moreover, a canister full signal is preferably generated in a
controller of the negative pressure wound therapy system once the
classification algorithm of the canister full detection method
detects a canister full condition. The canister full signal may be
immediately communicated to the user of the npwt system, for
example by means of an acoustic and/or visual alarm.
The canister full detection method may also comprise a step of
eliminating the canister full condition once the classification
algorithm detects a canister full condition. To eliminate the
canister full condition, the user of the negative pressure wound
therapy system simply has to replace the canister by a new, empty
canister.
Leakage Detection Method
According to another preferred embodiment of the invention, the
method according to the first aspect of the invention further
comprises determining a leakage condition of a negative pressure
wound therapy system. The method, which is designated in the
present specification as the "leakage detection method", comprises
the following steps: i. controlling an electrical pump for
generating a negative pressure, ii. generating a leakage signal if
a pump speed associated with the electrical pump exceeds a
predetermined value.
The leakage signal is usually generated in a controller of the
negative pressure wound therapy system. The leakage signal may be
immediately communicated to the user of the npwt system, for
example by means of an acoustic and/or visual alarm. Alternatively,
the signal may be communicated to the user of the npwt system with
a delay, for example with a delay of 1 to 10 minutes.
Preferably, the predetermined value for the pump speed is selected
of the range of 1500 RPM to 10000 RPM. Even more preferably, the
predetermined value for the pump speed is selected of the range of
3000 RPM to 6000 RPM. In particular, the predetermined value is
approximately 3000 RPM or approximately 4900 RPM.
The predetermined value may also advantageously be selected such
that the negative pressure wound therapy system is still able to
essentially maintain a desired target negative pressure when the
leakage signal is generated. This may be achieved by selecting a
pump speed threshold (predetermined value) as suggested
previously.
When using the "leakage detection method" it is further preferred
to maintain the pump speed at a predetermined constant value after
the leakage signal has been generated. For example, the pump may
maintain a speed of 4900 RPM after a leakage signal has been
generated at this threshold value. Alternatively, it is also
possible to deactivate the electrical pump after the leakage signal
has been generated. Both versions may prevent an increase of the
pump speed after detection of the leakage condition, which may save
electrical power. Moreover, both versions may prevent the
electrical pump to reach a too high operating noise.
Preferably, the leakage condition is eliminated after a leakage
signal has been generated. To eliminate the leakage condition, the
user of the negative pressure wound therapy system possibly has to
reseal the wound dressing.
Flow Rate Estimation Method
Proceeding to another preferred embodiment of the invention, the
method according to the first aspect of the invention further
comprises estimating a flow rate of a negative pressure wound
therapy system. The method, which is designated in the present
specification as the "flow rate estimation method", comprises the
following steps: i. controlling an electrical pump for generating a
negative pressure, ii. estimating the flow rate as a flow rate
function of a pump speed and a pump current.
Any mathematical equation for estimating the flow rate is a "flow
rate function" according to the invention as long as the equation
comprises the variable pump speed (or a variable derived from the
pump speed) and the variable pump current (or a variable derived
from the pump current).
Negative Pressure Wound Therapy System
The control methods according to the present invention are
described in more detail in exemplary fashion in the form of
negative pressure wound therapy systems. In principle, the
components and the general structure of negative pressure wound
therapy systems are known in the prior art, for example from patent
publications DE 10 2009 038 130 A1, DE 10 2009 038 131 A1 and DE 10
2011 075 844 A1 of the assignee. The features of the negative
pressure wound therapy systems described in the following examples
may also be included in a negative pressure wound therapy system
according to the second aspect of the invention.
The method according to the first aspect of the invention is
preferably performed by using a negative pressure wound therapy
system, which comprises an electrical pump for generating negative
pressure, optionally a tachometer for determining a pump speed
associated with the electrical pump, a pressure sensor for
determining negative pressure values, a controller for controlling
the activity of the electrical pump, input means for adjusting
settings on the negative pressure wound therapy system, said input
means being operable by the user of the negative pressure wound
therapy system, a first fluid path fluidly connectable to a wound
site and to the electrical pump such that the wound site can be
subjected to a negative pressure, wherein the pressure sensor is
located in the first fluid path between the wound site and the
electrical pump.
In particular, the electrical (actuated) pump is a membrane pump.
Membrane pumps suitable for negative pressure wound therapy systems
are commercially available, for example, from the company Schwarzer
Precision (Essen, Germany).
Suitable pressure sensors for the npwt system are marketed amongst
others by the company Freescale Semiconductor (Eindhoven,
Netherlands; e.g. pressure sensor MPXV2053DP).
The controller typically regulates the negative pressure wound
therapy system such that the negative pressure wound therapy system
executes the control methods disclosed in the present
specification. The controller may comprise a processor (CPU) and a
memory to record electronic data.
Preferred input means for adjusting settings on the negative
pressure wound therapy system is a touch screen.
The negative pressure wound therapy system may further have the
following additional features:
Preferably, the negative pressure wound therapy system comprises a
canister for collecting liquid from the wound site. The canister is
located in the first fluid path between the wound site and the
electrical pump. The pressure sensor is located in the first fluid
path between the canister and the electrical pump. A suitable
canister is disclosed, for example, in the international patent
applications WO 2014/177544 A1 and WO 2014/177545 A1.
Preferably, the negative pressure wound therapy system further
comprises means for preventing liquid from entering the electrical
pump, for example a moisture sensitive filter or a liquid
impermeable membrane. Said means for preventing liquid from
entering the electrical pump is located in the first fluid path
between the canister and the pressure sensor.
It is also preferred that the negative pressure wound therapy
system comprises a relief valve for venting the negative pressure
wound therapy system, wherein the relief valve can be controlled by
the controller, a second fluid path fluidly connectable to the
wound site and the relief valve, wherein the first fluid path and
the second fluid path are in fluid communication at the wound
site.
For practical purposes it has been found useful to include the
electrical pump, the tachometer (if present), the pressure sensor,
the controller, the input means, and the relief valve in a portable
negative pressure device. The means for preventing liquid from
entering the electrical pump should be included in the canister,
said canister being removably attachable to the negative pressure
device.
Preferably, the portable negative pressure device including the
canister is fluidly connectable to the wound site by means of a
suction conduit and a venting conduit. The suction conduit
constitutes a part of the first fluid path. The venting conduit
constitutes a part of the second fluid path.
If feasible, any of the aforementioned preferred methods,
embodiments or advantageous features may be used in combination
with each other. Any of said combinations may be used for a
negative pressure wound therapy system capable of performing a
method according to the first aspect of the invention. For example,
a method according to the first aspect of the invention may further
include the first or second pressure control method, any one of the
disclosed blockage detection methods, the canister full detection
method and the leakage detection method. Such a control algorithm
for a negative pressure wound therapy device would be capable of
controlling the pump activity in order to achieve the desired
negative pressure at the wound and to detect certain alarm
situations which may occur during the negative pressure wound
therapy.
The different control methods disclosed herein may also establish a
favorable interaction rather than being executed by the controller
independently from each other. Preferred interactions of the
different control methods disclosed in the present specification
are listed below.
The first or second pressure control method may be used to control
the speed of the electrical pump in step i. of the wound pressure
estimation method. control the speed of the electrical pump in step
i. of the first blockage detection method. control the speed of the
electrical pump in step i. of the second blockage detection method.
control the speed of the electrical pump in step vi. of the second
blockage detection method. control the speed of the electrical pump
in step i. of the canister full detection method. control the speed
of the electrical pump in step i. of the leakage detection method.
control the speed of the electrical pump in step i. of the flow
rate estimation method.
The wound pressure estimation method may be applied to all measured
negative pressure values used for the first pressure control
method. the negative pressure value in step ii. of the first
pressure control method. all measured negative pressure values used
for the second pressure control method. the negative pressure value
in step ii. of the second pressure control method. the negative
pressure values used to determine the actual negative pressure
gradient in step v. of the second pressure control method. the
first and the second negative pressure value (according to a
preferred embodiment of the second pressure control method) used to
determine the actual negative pressure gradient in step v. of the
second pressure control method. all measured negative pressure
values used for the first blockage detection method. all measured
negative pressure values used for the second blockage detection
method. all measured negative pressure values used for the canister
full detection method. the plurality of negative pressure values in
step ii. of the canister full detection method.
In particular, the first and second blockage detection method may
interact with each other. The interactive blockage detection method
may comprise the following steps: i. generating a negative pressure
at a wound site by means of an electrical pump, ii. recording the
negative pressure, iii. venting the fluid path of the negative
pressure wound therapy system by opening a relief valve, wherein
during the ventilation the electrical pump is stopped, iv.
monitoring and recording a negative pressure drop (or negative
pressure gradient) during the ventilation step, v. if the negative
pressure drop observed during the ventilation step within a
predetermined period of time is less than a predetermined negative
pressure drop (value), generating a blockage signal in a controller
of the negative pressure wound therapy system, vi. if the negative
pressure has dropped by the predetermined value within the
predetermined period of time, closing the relief valve when the
negative pressure has dropped by the predetermined value to
finalize the ventilation step, followed by the steps of vii.
determining and recording a negative pressure gradient occurring
during the entire ventilation step (average negative pressure
gradient during the ventilation step), viii. reactivating the
electrical pump to reestablish the negative pressure of step ii.,
ix. determining and recording a number of pump turns, which are
required to reestablish the negative pressure of step ii., x.
generating a first or a second blockage detection data set, said
first or said second blockage detection data set comprising the
recorded negative pressure of step ii., the recorded negative
pressure gradient occurring during the entire ventilation step and
the recorded number of pump turns, which were required to
reestablish the negative pressure of step ii., xi. executing a
classification algorithm which allows to discriminate a first
blockage detection data set, said first blockage detection data set
being correlated to an unblocked condition of the negative pressure
wound therapy system, from a second blockage detection data set,
said second blockage detection data set being correlated to a
blocked condition of the negative pressure wound therapy system,
xii. optionally generating a blockage signal in the controller of
the negative pressure wound therapy system, if a blocked condition
of the negative pressure wound therapy system is detected by means
of the classification algorithm.
The additional features of preferred embodiments of the first and
the second blockage detection method may also be implemented in the
interactive blockage detection method.
FIGURES
Further characteristics, details, and advantages of the invention
result from the appended patent claims and from the drawings and
the following description of preferred embodiments of the
invention. The drawings show:
FIG. 1 A schematic drawing of a simple negative pressure wound
therapy device including the negative pressure bandage applied to a
wound of a patient.
FIGS. 2 a to e Different views of a typical portable negative
pressure wound therapy device to generate a negative pressure for
medical applications.
FIG. 3 A schematic drawing of the piping system and of the
electronic components of a typical negative pressure wound therapy
device.
FIGS. 4 a and b The first function according to a preferred
embodiment of the invention.
FIGS. 5 a and b The second function according to a preferred
embodiment of the invention.
FIG. 6 The second pressure control method in a schematic overview
according to a preferred embodiment of the invention.
FIG. 7 The negative pressure in a npwt system during the blockage
detection method according to a preferred embodiment of the
invention.
FIGS. 8 a to c The blockage detection function as a part of a
preferred embodiment of the invention.
FIG. 9 The canister full detection function as a part of a
preferred embodiment of the invention.
FIG. 10 Experimental results concerning the leakage detection
method according to a preferred embodiment of the invention.
FIGS. 11 a to c Experimental results concerning the flow rate
estimation method according to a preferred embodiment of the
invention.
DESCRIPTION OF THE FIGURES
A simple negative pressure wound therapy device 1, which is in
fluid communication with a wound 2 of a patient to be treated is
shown in FIG. 1 schematically. Wound therapy devices of this type
are known in the prior art. In many cases, like the one shown in
this non-limiting example, the portable negative pressure wound
therapy device 1 has a container 3 adapted for receiving body
fluids, in particular wound exudates extracted from the wound by
suction. The container (or canister) 3 is typically made of a solid
material, such as a plastic material. It is usually a disposable
article designed for single use. Conveniently, the container 3 can
be detachably mounted to the housing part 4 of the device, which
contains the electrical and/or electronic components of the
apparatus. The container 3 can be evacuated by the electrically
actuated suction pump 5. A connection (not shown) is provided for a
suction line 6 that leads to the wound such that negative pressure
communication can be established between the suction pump 5, the
container 3, and the suction line 6 that leads to the wound. A
filter or air/liquid-separator 7 located within the fluid-pathway
between the container 3 and the suction pump 5 is used to prevent
exudate from being sucked into the pump 5. A negative pressure
wound therapy device typically comprises additional components such
as a control system for controlling activity of the pump and means
for interacting with the user, such as a touch-screen display or
control buttons. These components are not shown in FIG. 1.
In some embodiments, the portable negative pressure wound therapy
device does not have a container for receiving the drained body
fluids. Instead, the body fluids can be contained, for example, in
the dressing. This is achieved by providing absorbent layers (not
shown in FIG. 1). Such negative pressure wound therapy devices,
which do not make use of a separate solid exudate canister are
typically used for treating less exudating wounds, for example
surgical wounds.
FIGS. 2 a to e show a typical example of a portable device 1 for
the provision of the negative pressure for medical applications.
The device 1 comprises a first housing part 4 in which a negative
pressure-producing device in the form of an air suction pump 5 and
electrical and electronic control components for the device are
accommodated completely, including batteries or preferably
rechargeable batteries. A recharging connection for the batteries
is designated by reference symbol 8. Moreover, the device 1
comprises a second housing part that is also a container 3 for
receiving body fluids, in particular, for receiving wound exudates
suctioned away from a wound. The entire second housing part is
preferably constituted as a disposable single-use item. In its
upper region, a connection gland 9 for a suction tube is provided
that may, for example, lead to a wound dressing that sealingly
closes the wound when the device 1 is used in the negative pressure
therapy of wounds and there it can, for example, communicate with
the wound space through a port to apply and maintain a negative
pressure to the wound space and to suction away wound exudates into
the container. For this purpose, the container 3 communicates with
the suction pump 5.
It can also be seen from FIG. 2 d on the side 10 of the second
housing part 3 facing the body, a grip recess 11 is formed in the
shape of an opening extending right through the second housing part
3. In this way, the device 1, or only its second housing part 3,
can be gripped and handled with one hand.
In the preferred embodiment shown, a manually operable element 12
is provided in this grip recess 11 on the upper side of the device
1, for example, in the form of a pushbutton that acts on locking
and back-gripping means (not shown). In the joined condition of the
two housing parts 3 and 4, the locking or back-gripping means are
in a locked condition holding the two housing parts 3, 4 together
by positive action. Only on operation of the operating element 12,
the lock is released so that the housing parts 3, 4 can be
separated from each other.
FIG. 3 shows the nature of the piping system and of the electronic
components of an exemplary negative pressure wound therapy device,
for which the inventive control method can advantageously be used.
The device is similar to the negative pressure wound therapy device
of the type exemplified in FIG. 2. In contrast to the very basic
system shown in FIG. 1, the device of FIG. 3 includes additional
components (known from the art) such as the air rinsing pathway of
the fluid system. FIG. 3 shows the previously described device for
providing a negative pressure for medical applications in a purely
schematic representation, wherein relevant reference symbols are
used for the corresponding components. However, FIG. 3 only shows
those components that are relevant for describing the present
invention. FIG. 3 shows a wound to be treated (schematically) with
a negative pressure with a vacuum-tight wound dressing 13, to which
the suction tube 6 emanating from the container 3 leads. From the
container 3, a further tube section 14 leads outwardly through the
filter 7 mentioned previously. If the container 3 or the first
housing part 4 is put into its operating position on the first or
basic housing part 4 of the device 1, the tube section 14 is
connected to a further tube section 15 within the first housing
part 4 that leads to the intake side of the suction pump 5. When
the suction pump 5 operates, a negative pressure is applied to the
container 3 and to the suction tube 6 via tube sections 14, 15, and
air suctioned in from there is blown out to the environment via
outlet tube 16, wherein additionally non-depicted sound damping
elements and, if necessary, further filters can be provided.
Moreover, a pressure sensor 17 for measuring the pressure is
provided in the tube section 15 between container 3 and suction
pump 5. Its signals are sent to an electronic control unit 18,
which performs open-loop and closed-loop control of the device 1 in
total. The electronic control unit 18 comprises a microelectronic
controller and at least one electronic memory. Also shown is the
charging connection 8 for rechargeable batteries that are located
in a compartment 19 and a connection 20 for a schematically
indicated power supply unit 21. Reference symbol 22 indicates a
display unit, preferably having a capacitive switch membrane
(touchscreen). A user may control operation of the device via said
touchscreen. The electrical connection to the electronic control
unit 18 is only shown via electrical lines 23. The suction pump 5
is controlled by the electronic control unit 18 by means of the
signals of the pressure sensor 17, so that the pressure value
corresponding to the currently selected program is controlled in
the tube section 15.
Also shown is an additional rinsing or aeration tube 24 that
(according to an exemplary design) proceeds through the container 3
and just like the suction tube 6 leads to the wound dressing 13.
When the container 3 is attached in its intended assembly position
on the first housing part 4, this rinsing tube 24 communicates with
a tube section 25 provided in the first housing part 4. The first
housing part 4 comprises an electromagnetically operated valve 26
that can be actuated by the electronic control unit 18. Said valve
26 connects the tube section 25 with the atmospheric air when it is
open, so that an air current toward the wound via the rinsing tube
24 can be generated.
The device 1 and its electronic control unit 18 also feature a data
interface 27 (preferably a USB interface). The electronic control
unit 18 can be programmed using said data interface 27. In
addition, device 1 comprises a speaker 28 which is connected to the
control unit 18. The speaker can be used to generate acoustic alarm
signals. A user may set a target negative pressure via user
interface 22. After starting the therapy a negative pressure value
is determined by means of the pressure sensor 17. Pressure sensor
17 is located in a fluid path between the wound site 2 and the
electrical pump 5. The electrical suction pump 5 is used for
generating the negative pressure. The methods of generating a
negative pressure according to aspects or preferred embodiments of
the invention include calculating a difference between the negative
pressure value determined by the sensor 17 and the target negative
pressure setting to obtain a negative pressure error. As a
consecutive step a target negative pressure gradient is derived by
means of a first function. The first function maps the negative
pressure error to the target negative pressure gradient. Finally a
control signal is adjusted in response to the value of the target
negative pressure gradient. The control signal thus obtained is
used for controlling the speed of the electrical pump 5.
In the following, the novel methods for controlling a negative
pressure wound therapy system are explained in more detail (FIG. 4
to FIG. 11). These control methods represent particularly important
aspects of the present invention or preferred embodiments thereof.
The control methods disclosed in the present specification are
particularly suited for a negative pressure wound therapy system
with a general structure as shown in FIG. 2 and FIG. 3. However,
the control methods disclosed in the present specification may also
be suited for other negative pressure wound therapy systems.
Method of Generating a Negative Pressure at a Wound Site (First and
Second Pressure Control Method) Basically, the negative pressure
wound therapy system is permanently determining the actual pressure
present at the pressure sensor. The collected pressure values may
preferably be modified by means of the "wound pressure estimation
method" as explained below. The controller of the negative pressure
wound therapy system then compares the determined pressure value
with the "target pressure" selected by the user. The difference
between the determined pressure value and the target pressure is
the "pressure error". The core of the pressure control is the
desired "target pressure gradient". The target pressure gradient is
derived from a function. The input of said function is the pressure
error. This function is herein also designated as "first function".
An example for a first function is shown in FIGS. 4 a and b.
The x-axis of the diagrams included in FIGS. 4 a and b represents
the pressure error (difference between the measured pressure and
the target pressure). The y-axis of the diagrams in FIGS. 4 a and b
represents the target pressure gradient. FIG. 4 b is an enlarged
view of the central part of FIG. 4 a. As can be seen in FIGS. 4 a
and b, the first function provides a linear target response with
respect to pressure error values between approximately -2 mmHg and
100 mmHg. Beyond this range, the response remains either constant
(pressure error >100 mmHg) or further increases (pressure error
<approximately -2 mmHg) with a "S"-shaped curve progression to a
maximum target pressure gradient of 10 mmHg/s. The first function
(as well as the second function explained below) cannot be
conveniently described by means of a single mathematical equation.
The first (and the second) function may at most be described by a
combination of several mathematical equations (functions).
A pressure error with a negative algebraic sign may be obtained if,
for example, the measured negative pressure amounts to 115 mmHg and
the target negative pressure amounts to 125 mmHg (the pressure
error then amounts to -10 mmHg). In this case the npwt system has
not yet achieved the target negative pressure. A pressure error
with a positive algebraic sign may be obtained if, for example, the
measured negative pressure amounts to 135 mmHg and the target
negative pressure amounts to 125 mmHg (the pressure error then
amounts to 10 mmHg). In this case too much negative pressure is
present within the npwt system. In general, a target negative
pressure gradient above 0 (>0) may cause an increased pump
activity. Instead, a target negative pressure gradient below 0
(<0) generally may cause a decreased pump activity. The target
negative pressure gradient for pressure error values exceeding -100
(for example -110) in the shown example will always amount to 10
mmHg/s. Similarly, the target negative pressure gradient for
pressure error values exceeding 100 (for example 110) in the shown
example will always amount to -100 mmHg/s.
The target pressure gradient taken from the first function is then
compared with the actual pressure gradient yielding the "pressure
gradient error". The actual pressure gradient is based on the
pressure data received by the pressure sensor (which are preferably
modified by the "wound pressure estimation method" as already
mentioned). The pressure gradient error is the input for another
function, which allows for calculating the so called "integrator
input". This function, designated herein also as "second function"
and exemplarily depicted in FIGS. 5 a and b, is mainly an
adaptation and limitation of the signal, which finally controls the
pump activity. The second function therefore provides a weighting
to the integrator input based on the pressure gradient error.
The x-axis of the diagrams in FIGS. 5 a and b relates to the
pressure gradient error (difference between the measured pressure
gradient and the target pressure gradient). The y-axis of the
diagrams in FIGS. 5 a and b represents the integrator input. The
second function shown in FIG. 5 a exhibits a flat "S"-shaped curve
progression in the pressure gradient error range of approximately
-35 mmHg to 35 mmHg. FIG. 5 b shows the previous second function
together with three alternative versions of the second function
having narrower "S"-shaped sections. The pressure control method
may include only one of the shown second functions. However,
adapting the second function in the course of the negative pressure
wound therapy may reduce oscillations in the generated pressure
and, therefore, further improve the pressure control method. For
example, the controller of the npwt system may adapt the second
function during the cycles of the pressure control method based on
the magnitude of the pressure gradient fluctuations. Thus,
depending on the magnitude of the pressure gradient fluctuations,
the controller determines a particular suited second function
adapted to the current circumstances which may look like one of the
functions in FIG. 5 b (or at least look similar to the functions in
FIG. 5 b).
A pressure gradient error with a positive algebraic sign may be
obtained if, for example, the measured negative pressure gradient
amounts to 1 mmHg/s and the target negative pressure gradient
amounts to 2 mmHg/s (the pressure gradient error then amounts to 1
mmHg/s). In this case the npwt system has not yet achieved the
target negative pressure gradient. A pressure gradient error with a
negative algebraic sign may be obtained if, for example, the
measured negative pressure gradient amounts to 3 mmHg/s and the
target negative pressure gradient amounts to 2 mmHg/s (the pressure
gradient error then amounts to -1 mmHg). In this case the negative
pressure in the npwt system increases too fast. In general, an
integrator input value above 0 (>0) may cause an increased pump
activity. Instead, an integrator input value below 0 (<0)
generally may cause a decreased pump activity. The integrator input
for pressure gradient error values exceeding -40 (for example -50)
in the shown examples will always amount to -0.5. Similarly, the
integrator input for pressure gradient error values exceeding 40
(for example 50) in the shown examples will always amount to
0.5.
The integrator output may already constitute the control signal for
the pump. Alternatively, the integrator output may be transformed
(or "translated") into the final control signal for the pump. Said
final control signal for the pump may be for example, the pump
voltage (signalling voltage of the pump). There may exist a third
or even further functions (not shown on the figures), which
transforms the integrator output to the final control signal (e.g.
the pump voltage) and/or further adapts the integrator
output/control signal in accordance with certain pump
characteristics. However, such a third or further function is not
necessarily required.
The suggested pressure control algorithm effectively works as a PID
controller using the target pressure gradient instead of the
pressure as its primarily input.
The first function is the most important one, because it has a
predominant influence on the general control performance of the
pressure controller. The second function and the third function add
performance improvements. By using the pressure control method
suggested in the present specification, the npwt system may be able
to generate and maintain the desired target negative pressures
effectively but at the same time smoothly. Smooth pressure
adaptations during therapy improve patient comfort.
An outline of the pressure control method for generating a negative
pressure at a wound site is given in FIG. 6.
Method of Estimating a Negative Pressure at a Wound Site (Wound
Pressure Estimation Method)
The objective of the wound pressure estimation method is to compute
a modification value which may be used to compensate for a pressure
drop appearing between a pressure sensor located near a negative
pressure source (pump) and a wound site. During experiments
performed using a wound simulator it was unexpectedly found that
the pressure drop is proportional, at least to a great extent, to
the pump speed. It was also found that said pressure drop is (at
least to a great extent) independent of the pressure present at the
pump. It is therefore possible to get a highly reliable estimation
of the pressure drop by multiplying pump speed by a constant
value:
"Modification Value Formula" modification value (mmHg) [i.e.
pressure drop]=constant (mmHg/RPM).times.pump speed (RPM)
The constant has to be determined empirically for each type of npwt
system.
The estimated pressure drop (modification value) may then be used
to estimate the pressure present at the wound:
"Pressure Estimation Formula" estimated negative pressure at the
wound (mmHg)=measured negative pressure (mmHg)-(constant
(mmHg/RPM).times.pump speed (RPM))
The abbreviation RPM stands for "revolutions per minute" and is the
unit of the pump speed. Typically, the pump speed is measured from
the output of the pump tachometer.
In summary, the wound pressure estimation method is based on a
modification value applied to the pressure data received from the
pressure sensor. The pressure modification compensates for the
estimated pressure drop between a pressure sensor located near a
pressure source and the wound. Advantageously, the wound pressure
estimation method is working continuously while the negative
pressure wound therapy system is active, except during flushing
(venting).
The following example illustrates application of the wound pressure
estimation method by referring to the npwt systems shown in FIG. 1
and FIG. 3:
A negative pressure value of 125 mmHg is determined using pressure
sensor 17. The pressure sensor is located in the fluid path between
pump 5 and filter 7. The pump speed of the electrical pump 5 at the
time of the pressure measurement is 1000 RPM. The constant
determined for the npwt system used for the experiments is 0.0075
mmHg/RPM. Using the "pressure estimation formula" disclosed herein,
the estimated negative pressure at the wound site 2 is 117.5 mmHg:
estimated negative pressure at the wound (mmHg)=125 mmHg-(0.0075
mmHg/RPM.times.1000 RPM)=117.5 mmHg
The example demonstrates that the negative pressure measured near
the negative pressure source is usually higher than the negative
pressure actually applied to the wound site. Treating the wound at
an incorrect negative pressure level may impair the efficacy of the
negative pressure wound therapy.
Method of Determining a Blockage Condition in a Negative Pressure
Wound Therapy System (First and Second Blockage Detection
Method)
The blockage detection method of the negative pressure wound
therapy system necessarily incorporates a flush (venting)
procedure. Thus, the blockage detection method may advantageously
be used for an npwt system having a separate fluid path for
performing a venting procedure (such as the npwt system described
in FIG. 3). The blockage detection method acts independently of the
canister full detection method. The blockage detection method
suggested in the present specification is versatile and works
precisely and reliably. Moreover, the disclosed method is easy to
perform once the classification algorithm has been established.
The blockage detection method according to a particularly preferred
embodiment comprises the following steps: "Pressure
generation/stabilise": Regulate the negative pressure wound therapy
system to a negative pressure, for example to the target negative
pressure. Advantageously, the negative pressure to which the system
is regulated is a "stable negative pressure". A stable negative
pressure is present if, for example, the following two conditions
i) and ii) are met:
i) The negative pressure exceeds a certain value, for example a
value of 18.6 mmHg.
ii) The pressure gradient remains within a certain (narrow) range,
for example within the range of -1 mmHg/s and 1 mmHg/s or within
the range of -0.5 mmHg/s and 0.5 mmHg/s. A stable negative pressure
may also be defined by different requirements. Regulating the
pressure to a stable negative pressure is the object of a preferred
embodiment, where it may further improve reliability of the
blockage detection. Nevertheless, the stable negative pressure is
not necessarily required to perform the blockage detection method.
"Evacuate/venting": Record the start pressure, open the relief
valve and stop the pump. Record the pressure gradient until the
pressure drops by 20% or until a 45 second timeout elapses.
"Recover & Hold": Close the relief valve and restart the pump
in order to return to the pressure, which has been recorded at the
start of the evacuation step. Record the number of pump turns.
"Evaluate blockage score": Evaluate a blockage score using (1) the
recorded pressure at the start of the evacuate step, (2) the
average pressure gradient during the evacuate step, and (3) the
number of pump turns during the recover & hold step (the three
variables form a blockage detection data set).
If the 45 second timeout elapses before the pressure drops by 20%
during the evacuate step, the blockage detection method is
terminated and a tube blockage signal is set (the alarm signal,
however, is preferably only released after the tube blockage is
finally verified, see below).
If the pressure drops by 20% within 45 seconds during the
evacuation step (leading to a regular termination of the flush
procedure), the blockage detection data set is evaluated. Said
evaluation is done using a linear function which describes a plane
in 3D space that separates "blocked" points (second blockage
detection data sets) from "unblocked" points (first blockage
detection data sets) derived from the aforementioned variables (1),
(2) and (3). An exemplary blockage detection function is depicted
in FIGS. 8 a to c. If the evaluation results in a detection of a
blockage condition, a tube blockage signal is set.
The tube blockage detection method may be active, for example,
every five minutes. When a tube blockage signal is set, the tube
blockage detection method is preferably repeated after two minutes
to re-evaluate the blockage condition. If the tube blockage is
verified, an alarm is displayed to the user. In this example, a
user receives the alarm not later than 7 minutes after the blockage
initially appeared. The alarm informs the user that a blockage
condition exists in the negative pressure wound therapy system. The
user may then initiate the necessary steps to eliminate the
blockage condition, for example by replacing the suction conduit
being clogged with wound exudate.
FIG. 7 shows an example of the negative pressure curve in a npwt
system during the blockage detection method (schematic
representation). The x-axis represents time (t), the y-axis
represents negative pressure (P). In this example, the npwt system
generates a stable negative pressure P.sub.S of 100 mmHg. The
stability of the negative pressure is schematically indicated in
FIG. 7 by the straight pressure curve (parallel to the x-axis)
prior to time t.sub.1. At time t.sub.1, the ventilation step is
initiated by opening the relief valve and at the same time stopping
the pump of the npwt system. By opening the valve, air enters into
the fluid path leading to a negative pressure decrease such that
the pressure curve in FIG. 7 declines. After a pressure drop of 20%
(that is when the negative pressure is at 80 mmHg (P.sub.D)), the
relief valve closes. Closure of the relief valve occurs at time
t.sub.2. Subsequently, the npwt system re-establishes the negative
pressure, which was present at the beginning of the ventilation
step (i.e. 100 mmHg in this example). Therefore, the pressure
increases between t.sub.2 and t.sub.3. Starting with time t.sub.3
the npwt system is on a negative pressure level of 100 mmHg.
P.sub.R in FIG. 7 stands for the negative pressure at time t.sub.3.
Pressure P.sub.R is equal to (corresponds to) pressure P.sub.S. Any
first or any second blockage detection data set in this example is
derived from the parameters P.sub.S, P.sub.D, the pressure gradient
(between t.sub.1 and t.sub.2) and the number of pump turns (between
t.sub.2 and t.sub.3).
The method for determining a blockage condition in a negative
pressure wound therapy system during a negative pressure wound
therapy disclosed herein includes a classification algorithm. In
principle, a classification algorithm is used to decide, if an
individual event belongs to a first or to a second class of events.
In order to establish a classification algorithm a high number of
experiments ("training experiments") has to be done to generate a
plurality of events corresponding to one of the two classes (for
example 50 experiments of events belonging to the first class and
50 experiments of events belonging to the second class).
Furthermore, it is necessary to establish criteria which are used
to discriminate the two classes. It is possible to represent the
single events by entering each event into an n-dimensional data
space. Each data point represents an individual event characterized
by n parameters. If each of the two classes form an interconnected
group of data (in the n-dimensional space), which does not overlap
with the other class, it is possible to discriminate the groups by
using a (n-1)-dimensional separator. The separator is also called
hyperplane. If the data space is 3-dimensional, the hyperplane is a
plane. If the data space is 2-dimensional, the hyperplane is a
line. The hyperplane can be constructed "manually". Preferably, the
hyperplane is established by using a support vector machine. FIGS.
8 a to c exemplary show training experiments required to establish
a separation plane (hyperplane). Said hyperplane is used to perform
a blockage detection method as described herein. In other words:
The hyperplane is used as a blockage detection function.
FIGS. 8 a to c show the separation plane (blockage detection
function) from different perspectives. The figures provide an
example of a three-dimensional space (coordinate system) and a
separation plane, which can be used for the blockage detection
classification algorithm. The x-axis of the diagrams represents
values derived ("transformed") from the number of pump turns (i.e.
the number of pump turns were put in relation to the pressure drop
(P.sub.S-P.sub.D)). The y-axis of the diagrams represents values
derived ("transformed") from the pressure gradient (i.e. the
pressure gradient was put in relation to
0.5.times.(P.sub.S+P.sub.D)). Finally, the z-axis of the diagrams
represents the start pressure. In this case the negative pressure
values represented by the z-axis are provided with negative
algebraic signs. The diagrams in FIGS. 8 a to c also show the
blockage detection data sets that were generated as a result of a
plurality of blockage detection training experiments. Each data
point in the coordinate system corresponds to a blockage detection
data set. The circles in the diagrams indicate first blockage
detection data sets each representing an unblocked condition. The
triangles in the diagrams indicate second blockage detection data
sets each representing a blocked condition. As can be seen in the
diagrams, the first and the second blockage detection data sets are
forming classes which do not overlap with each other. It is
possible to separate the first from the second class by a
2-dimensional plane. The calculation of the separation plane shown
in FIGS. 8 a to c was done by using a standard support vector
machine. The separation plane provides a measure whether any
individual future blockage detection event (represented by a
blockage detection data set), which is the result of performing the
blockage detection method disclosed herein, corresponds to an
unblocked condition (first class) or to a blocked condition (second
class). All data points located above (to the right of) the
separation plane are classified as an unblocked condition (first
class) of the examined negative pressure wound therapy system. In
contrast, all data points located underneath (to the left of) the
separation plane are classified as a blocked condition (second
class) of the examined negative pressure wound therapy system. In
FIG. 8 a, two arrows indicate the direction of "above/to the right
(a/r)" and "underneath/to the left (u/l)" in connection with the
separation plane.
To generate the blockage detection data sets shown in FIGS. 8 a to
c, a negative pressure wound therapy system as described in
connection with FIG. 2 and FIG. 3 was experimentally subjected to a
series of unblocked and to a series of blocked conditions. The
experiments included the use of the wound simulator device
basically as disclosed in the international application WO
2010/072349 A1 of the applicant. To generate negative pressure, the
tested negative pressure wound therapy system used the membrane
pump SP622 EC-BL of the company Schwarzer. Furthermore, the tested
negative pressure wound therapy system executed the aforementioned
pressure control method (first and second pressure control method)
to control the pump. The negative pressure measurements for the
start pressure and the pressure gradient as well as the number of
pump turns (revolutions) according to the aforementioned blockage
detection method were recorded during the experiments. Moreover,
the blockage condition was determined during the experiments. In
this way the experimentally determined data points could be
assigned to either a blocked condition or to an unblocked
condition.
FIGS. 8 a to c only provides an example for a blockage detection
function (hyperplane), which was determined for a particular
negative pressure wound therapy system. If the blockage detection
method should be applied to another negative pressure wound therapy
system, it may be necessary to repeat the experiments and to
calculate a new blockage detection function.
Method of Determining a Canister Full Condition in a Negative
Pressure Wound Therapy System (Canister Full Detection Method)
In principle, detection of a canister full status (blocked canister
port/filter) is based on monitoring the pressure at the pump and
pump speed over time. It is preferred that the canister full
detection method runs continuously while negative pressure wound
therapy is active. The canister full detection method is in
particular designated for an npwt system comprising a moisture
sensitive filter in the fluid path between the canister and the
pressure sensor (such as the npwt system described in FIG. 3). The
canister full detection method works independently of the tube
blockage detection method. The canister full detection method
suggested in the present specification is robust and works
precisely and reliably. Moreover, the disclosed method is easy to
perform once the classification algorithm has been established.
Similar to the blockage detection method explained previously, the
canister full detection method uses a classification algorithm to
discriminate a "canister full" from a "canister not full"
condition. The canister full detection method evaluates a score
based on two variables. Said variables are derived from the most
recent 3 seconds of pump speed history and pressure sensor data
point history:
1) The number of pump turns (revolutions) in the last 3
seconds.
2) A pressure variation score which represents the degree to which
the pressure has both increased and decreased over the last 3
seconds, derived from the product of pressure increments and
decrements over the period.
Preferably, the canister full detection method does not initiate
until sufficient information is available so that significant
results can be expected. Accordingly, the recorded data are first
checked to determine if there is sufficient information to
correctly evaluate whether the canister is full or not. For
example, if the pump has not turned a single revolution, then the
data do not comprise sufficient information. In such a situation
the algorithm will bypass evaluation until the conditions for a
significant evaluation are met.
The canister full detection method is evaluated using a linear
function which describes a line in 2D space that separates blocked
points from unblocked points according to the graph exemplarily
depicted in FIG. 9. If the evaluation results in a detection of a
canister full condition, an alarm may be generated by the negative
pressure wound therapy system to notify the user accordingly. The
user may then replace the full canister by a new one and continue
the negative pressure wound therapy.
FIG. 9 shows an example of a canister full detection function
(separation line or "hyperplane"; dotted straight line with
reference sign "a" in the diagram) in a two-dimensional space
(coordinate system). The separation line is required to perform the
classification algorithm included in the canister full detection
method according to aspects or preferred embodiments of the
invention. The x-axis of the diagram represents the number of pump
turns (revolutions). The y-axis of the diagram represents the
pressure variation score. The diagram in FIG. 9 also shows
experimentally determined canister full detection data sets (as
data points in the coordinate system) that were used for
calculating the separation line. The circles in the diagram
indicate data sets representing a canister not full condition
(encircled by line "c"). The entirety of circles forms the first
class of events each corresponding to a canister not full
condition. The triangles in the diagram indicate data sets
representing a canister full condition (encircled by line "b").
Accordingly, the entirety of triangles forms the second class of
events, each corresponding to a canister full condition. As can be
seen in the diagram, the first and the second class of canister
full detection data sets do not overlap with each other. The
calculation of the separation line includes the use of a standard
support vector machine. The separation line provides a measure
whether any individual future canister detection event (represented
by a canister full detection data set), which is the result of
performing the canister detection method disclosed herein,
corresponds to a canister not full condition (first class) or to a
canister full condition (second class). All data points located
above the separation line are classified as a canister full status
(second class) of the examined negative pressure wound therapy
system. In contrast, all data points located underneath the
separation line are classified as a canister not full status (first
class) of the examined negative pressure wound therapy system. For
example, if in the course of the canister full detection method 10
revolutions and a pressure variation score of 100 mmHg are
recorded, the corresponding data point would be located above the
separation line. Accordingly, a canister full status would be
recognised. If in the course of the canister full detection method
10 revolutions and a pressure variation score of only 20 mmHg are
recorded, the corresponding data point would be located underneath
the separation line. Thus, in this further example a canister not
full status is determined.
To generate the data sets shown in FIG. 9, a negative pressure
wound therapy system as described in connection with FIG. 2 and
FIG. 3 was experimentally subjected to a series of canister full
and to a series of canister not full conditions (the canister not
full conditions included only partially filled canisters as well).
The experiments included the use of the wound simulator device
basically as disclosed in the international application WO
2010/072349 A1 of the applicant. To generate negative pressure, the
tested negative pressure wound therapy system used the membrane
pump SP622 EC-BL of the company Schwarzer. Furthermore, the tested
negative pressure wound therapy system executed the aforementioned
pressure control method (first and second pressure control method)
to control the pump. The number of pump turns (revolutions) and the
negative pressure measurements to calculate the pressure variation
score according to the aforementioned canister full detection
method were recorded during the experiments. Moreover, the filling
degree of the canister was determined during the experiments. In
this way the experimentally determined data points could be
assigned to either a canister full status or to a canister not full
status.
As already pointed out, FIG. 9 only provides an example for a
canister full detection function, which was determined for a
particular negative pressure wound therapy system. If the canister
full detection method is used for other negative pressure wound
therapy systems, it may be necessary to perform training
experiments and to calculate a canister full detection
function.
Method of Determining a Leakage Condition of a Negative Pressure
Wound Therapy System (Leakage Detection Method)
It is preferred that the leakage detection method is applied
continuously while negative pressure wound therapy is active. The
leakage detection method does not make use of the output value from
the flow rate estimation. The leakage alarm is generated if the
pump speed exceeds a predetermined value (threshold), for example
3000 RPM as shown in the diagram included in FIG. 10. This means
that the red/green threshold (red=leakage condition; green=no
leakage condition) is at a constant pump speed. In FIG. 10, said
threshold is represented by the diagonal line. Consequently, the
leak flow rate which causes a red status (leakage condition) will
be higher as the target pressure decreases. This method has the
benefit that it keeps the wound pressure close to the target
pressure for as long as possible. Keeping the wound pressure close
to the target pressure for as long as possible is achieved across
the full pressure range. Also, the audio noise at the red/green
threshold ("handover point") will be about the same for any target
pressure. Having a more constant audio noise is more convenient for
the patient. The leakage detection method as disclosed herein may
be used in combination with the methods for controlling the speed
of the suction pump as described previously (i.e. the first and the
second pressure control method).
The results depicted in FIG. 10 were obtained by means of the
following experiments: A negative pressure wound therapy system as
previously described in connection with FIG. 2 and FIG. 3 including
an artificial wound (size: 240 cm.sup.3) is subjected to different
leakage conditions. The experiments include the use of the wound
simulator device as basically disclosed in the international
application WO 2010/072349 A1 of the applicant. This wound
simulator device comprises the aforementioned artificial wound. The
wound simulator device comprises a valve and a flow meter to create
and determine the leakage condition of the tested npwt system. To
generate negative pressure, the tested negative pressure wound
therapy system used the membrane pump SP622 EC-BL of the company
Schwarzer. Furthermore, the tested negative pressure wound therapy
system executes the aforementioned pressure control method (first
and second pressure control method) to control the pump and to
generate the desired target negative pressure value.
The amount of air entering the fluid path of the negative pressure
wound therapy system is represented by the x-axis of the diagram in
FIG. 10. The y-axis represents the negative pressure within the
fluid path of the system. A higher leak flow rate corresponds to a
higher leakage condition of the system. During the experiment, a
target negative pressure value of approximately 200 mmHg is chosen
(line A) and it is studied how long the negative pressure wound
therapy system is able to maintain the desired target negative
pressure value. The experiment is repeated with a target negative
pressure value of approximately 125 mmHg (line B).
The inventors observed that the tested negative pressure wound
therapy system is able to maintain the desired target negative
pressure of 200 mmHg until the leak flow rate reaches a value of
approximately 2 I/min (line A). Thus, any leak flow rates above
approximately 2 I/min cannot be compensated by the pump contained
in the negative pressure wound therapy system anymore. However, if
the target negative pressure value is only 125 mmHg the negative
pressure wound therapy system is able to compensate a higher leak
flow rate, namely a leak flow rate up to approximately 2.5 I/min
(line B). Consequently, the leak flow rate causing an alarm
condition with regard to the negative pressure maintenance depends
on the selected target negative pressure. The inventors
unexpectedly found an advantageous and novel leakage detection
method. Said method comprises generating a leakage alarm, if the
pump speed exceeds a predetermined value. The methods considers the
observed dependency of the critical leak flow rate and the target
negative pressure. The diagonal line in FIG. 10 indicates when the
pump runs with a constant speed of 3000 RPM. As can be seen in the
diagram, an alarm is triggered (for example) at a leak flow rate of
1.5 I/min when the target negative pressure is 200 mmHg (line A).
As further can be seen in the diagram, an alarm is triggered at a
higher leak flow rate of approximately 2.1 l/min when the target
negative pressure is 125 mmHg (line B). However, in both cases the
negative pressure wound therapy system is still able to maintain
the desired target negative pressure when the alarm is triggered.
The same or a similar safety distance to the critical leak flow
rate is provided. In principle, it may even be possible for the
negative pressure wound therapy system of FIG. 10 to choose a
higher pump speed for the leakage detection method since the safety
distance to the critical leak flow rates could be further reduced.
In general, the pump speed threshold will essentially depend on the
type of the suction pump used (i.e. size and performance of
pump).
Method of Estimating a Flow Rate of a Negative Pressure Wound
Therapy System (Flow Rate Estimation Method)
It is preferred that the flow rate estimation is calculated
continuously while negative pressure wound therapy is active. The
flow rate is estimated as a function of pump speed and pump
current. Pump pressure is not used to estimate flow rate. It was
surprisingly found by the inventors that combining pump speed and
pump current provides a better estimate of flow rate than pump
speed alone (see FIGS. 11 a to c). For the flow rate estimation
method, the speed of the suction pump can be controlled for example
by means of the aforementioned pressure control method (first and
second pressure control method).
A negative pressure wound therapy system as previously described in
connection with FIG. 2 and FIG. 3 including an artificial wound is
subjected to different operating conditions (pressure, pump speed
and leak flow rate). The results of the experiments are shown in
FIGS. 11 a to c. The experiments include use of the wound simulator
device as basically disclosed in the international application WO
2010/072349 A1 of the applicant. This wound simulator device
comprises the aforementioned artificial wound. To generate negative
pressure, the tested negative pressure wound therapy system uses
the membrane pump SP622 EC-BL of the company Schwarzer.
Furthermore, the tested negative pressure wound therapy system
executes the aforementioned pressure control method (first and
second pressure control method) to control the pump and to generate
the desired target negative pressure values.
FIG. 11 a shows the graph of pump flow rate (as measured by an
additional flow rate sensor located on the pump's exhaust) vs. the
pump speed (as measured by the pump's tachometer). The four lines A
to D show how the pump flow rate is broadly linearly related to the
pump speed at constant pressure (constant negative pressure: line
A=20 mmHg; line B=65 mmHg; line C=125 mmHg; line D=200 mmHg).
However, the disparity/spread between the lines of constant
pressure means that if just the pump speed is used to estimate flow
rate (using a best fit polynomial estimator, shown as dotted line
in FIG. 11 a), then the worst case estimation errors are: 0.86
l/min absolute error and 37% relative error. During the
experiments, the pump current was also measured. FIG. 11 b shows
the corresponding graph of pump flow rate vs. pump current. The
inventors observed that there is likewise a dependency on pressure,
but the relationship between current and flow at constant pressure
is non-linear (i.e. not a straight line). Finally, FIG. 11 c shows
a linear regression based estimate of the flow rate vs. the
measured pump flow rate. The worst case estimation errors are: 0.24
l/min absolute error; 22% relative error (including flow rates 0.5
l/min); 10% relative error (excluding flow rates 0.5 l/min). These
estimation errors are significantly lower than the previously
mentioned estimation errors of FIG. 11 a (where the flow rate is
estimated based on the pump speed alone). In summary, the performed
experiments clearly show that the flow rate can be estimated very
well based on the variables pump speed and pump current. As known
in the prior art, the flow rate is a useful measure in negative
pressure wound therapy systems and can be used, for instance, in
control methods to detect a blockage condition or a leakage
condition.
The following formulas provide an example how the flow rate can be
mathematically derived from the pump current and the pump speed
according to the invention. DF stands for "density factor". The
density factor relates to the density of the air being evacuated by
the npwt system. PC is the measured pump current. PS is the
measured pump speed. Typically, PC and PS are measured at the same
time. DFA stands for "density factor adjustment" and provides a
mathematically modified density factor (DF) value. Finally, EFR
represents the "estimated flow rate". The units of pump current and
pump speed are Ampere (A) and revolutions per minute (RPM),
respectively.
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Wound Simulator Device/Experimental Setup
The inventors used a negative pressure wound simulator device to
develop the control methods disclosed in the present specification.
Such wound simulators are well-known. The general setup of the
experiments has been described above. This paragraph provides
supplemental information about the wound simulator and the
experiments.
The inventors built a wound simulator basically in accordance with
the wound simulator disclosed in WO 2010/072349 A1. The wound
simulator comprises a recess which serves as an artificial wound.
The artificial wound is connected to a source of liquid. Thus, a
liquid, for example a synthetic wound exudate solution, can be
introduced into the artificial wound. The npwt system to be tested
may then suck the synthetic wound exudate solution from the
artificial wound. The wound simulator comprises several sensors
including a pressure sensor located in the artificial wound. The
signals of this pressure sensor show whether the desired target
negative pressure in the wound space is established or not.
Furthermore, the wound simulator comprises a flow meter in
connection with a valve. The tested negative pressure wound therapy
system may be subjected to different leakage conditions by
(stepwise) opening the valve. The aforementioned flow meter
quantifies the leakage condition.
The inventors used commercially available dressing materials to
cover/fill the artificial wound. The dressing material included a
porous polyurethane foam to fill the artificial wound
(VivanoMed.RTM. Foam; Paul Hartmann, Heidenheim, Germany) and an
adhesive film (Hydrofilm.RTM.; Paul Hartmann, Heidenheim, Germany)
to seal the artificial wound. A multilumen conduit having a suction
and a ventilation lumen as well as a connector (VivanoTec.RTM.
Port; Paul Hartmann, Heidenheim, Germany) enabled fluid
communication between the dressing and the tested negative pressure
device. The tested negative pressure device had a general structure
as described in connection with FIG. 2 and FIG. 3. In some
experiments the tested negative pressure device included additional
or other components, for example a flow rate sensor located on the
exhaust of the pump to develop the flow rate estimation method. In
some experiments the controller of the negative pressure device was
supported or replaced by an external computer such as a laptop to
simplify data recording and processing.
During the experiments the negative pressure wound therapy system
was subjected to different operating conditions. The operating
conditions were chosen according to the particular purpose of the
experiments and included, for example, different target negative
pressure values, different sizes of the artificial wound or
different amounts of the synthetic wound exudate solution.
The leakage conditions were generated as explained previously (by
introducing different amounts of air into the artificial wound
space).
The blockage conditions were generated by repeatedly interrupting
fluid flow on different positions of the suction tube (for example,
at a position close to the artificial wound as well as at a
position remote from the artificial wound). Interruption of fluid
flow was done by bending the conduit or by using a clamp. In order
to verify that a blockage is actually present a flow meter
interposed in the fluid path was used. It was also possible to
inspect the flow with the naked eye by using a coloured synthetic
wound exudate solution.
Canister full conditions were simulated by introducing varying
amounts of coloured synthetic wound exudate solution into the
canister. When the liquid reached the lower edge of the filter the
canister was shaken softly so as to wet the filter completely. A
test condition was classified as a canister full condition as soon
as the filter was wet completely.
Mode of Pressure Sampling/Filter Technologies
According to a preferred embodiment, the therapy software module
(controller) for the negative pressure wound therapy system
continuously samples pressure measurements from the pressure sensor
at a rate of 100 samples per second. Preferably, permanent sampling
of pressure measurements is continued throughout the therapy
independent of any system conditions such as pump activity or
relief valve status. Inter alia, the pressure measurement values
are used for controlling negative pressure, for regulating air
flushes, for detecting tube blockages and for detecting a canister
full condition.
Preferably, the pressure values measured by the pressure sensor are
filtered in order to compensate for pressure fluctuations (noise
suppression). Noise suppression can be done using standard filter
technology such as digital filters (numerical implementation) or
analogue filters (electronic circuit). Similarly, the pump speed
measurements and pump current measurements may also be filtered in
order to compensate for fluctuations. In the present specification,
any reference to a pressure value measured by a pressure sensor may
therefore relate to a filtered pressure value. This also applies to
variables derived from pressure measurements such as the pressure
gradient, the pressure error, or the pressure gradient error.
Similarly, any reference to a pump speed measurement or to a pump
current measurement may therefore relate to a filtered pump speed
or to a filtered pump current, respectively.
* * * * *